The alluvial minerals of the River Indus, West Pakistan

Abstract

The river Indus originates at an elevation of 17,000 feet on the northern flank of Kailash Mountain in Tibet, and before it joins the Indian Ocean near Karaohi in West Pakistan, it flows for about 2000 miles, partly in the mountainous terrain of the Himalaya and partly over the plains of Sind and the Punjab. The investigations reported upon in this thesis relate to a study of the alluvial deposits in the upper reaches of the river, along a stretch of about 500 miles between Skardu in the Great Himalayas and Kalabagh in the Outer Himalayas. The thesis includes a description of the bed-rock geology of the area, and reports studies on the degree of sorting and the mineralogy of the gravels. Only the economic aspects of this work are reported in this abstract. Because of accessibility, the alluvials between Attock and Amb were chosen for economic study. Hero a primitive gold-washing industry exists and around 20 or 25 families are seasonably engaged in gold production. At a very rough estimate, the overall production is only 14 troy ounces per year, worth say £175. The method of mining, using a primitive sluice known as a nava, is described; and a report is given on detailed sampling tests employing a skilled gold-washer. The average yield is 1.05 grains per cubic yard of specially selected alluvium. The valuable mineral species present in the alluvium are gold, uraninite, tinstone and scheelite. The value of the average yield of gold from selected "high-grade" alluvium is about 60 per cubic yard. From the radiometric and mineralogical assays the tenor of uraninite in the natural sands in estimated to be around. 0,0002%, equivalent at full recovery to about Id per cubic yard. The tenor of tinstone and scheelite have been estimated visually to be about 1 oz. and 0.02 oz per cubic yard; but this estimate based on grain counts is almost certainly much too high for tinstone, since "tinning" tests using zinc and hydrochloric acid show that cassiterite is much rarer than the visual, optical, assessment. Considered overall, therefore, even the small patches of heavy mineral concentrate on which the indigenous gold industry is based have a value of contained minerals of well under one shilling per cubic yard. Since the grade of material which would have to be worked in a large-scale mechanized operation would be much lower than that of selected patches operated on manually, it is plain that there are no commercial prospects for any large-scale dredging. Radiometric studies have been conducted on the bed-rocks traversed by the river, and also on the alluvium. The highest values encountered are, firstly, in veins of aplite, pegmatite and younger granite giving 0.03- 0.55 mr/hr; secondly, graphitic schists usually giving 0.03 - 0.04 mr/hr, but rising in pockets to 0.08 mr/hr; and thirdly, some acid gneissose bands in the metamorphic formation giving as high as 0.15 mr/hr. Considered overall, no significant change in the radioactivity profile has been found along the course of the river; but the greatest proportion of high values is to be found where the country rocks are the metamorphosed gneisses bearing bands of high radioactivity. This suggests that the main source of the uraninite is local, predominately in the metamorphic rocks between Amb and Pattan, the mineral most probably occurring as disseminations of dispersed grains in these formations. In the terrace gravel deposits uraninite is less frequent at depth than it is near the surface; and in hand-panned concentrates from the Siwalik sandstones, which represent the alluvials derived from the Himalayan crystalline rocks by the Indus-Brahma river system of Neogene times, uraninite could not be found at all. These facts suggest that detrital uraninite does not survive lithification but is removed from sandstones by intra-stratal waters permeating the rooks during the period of early to late diagenesis.